Sickle Cell Disease (SCD) results from a single nucleotide mutation in hemoglobin that causes red cell sickling. It presents clinically as repeated occurrences of painful vaso-occlusive crises (i.e., pain crisis), acute chest syndrome, cerebrovascular accidents, splenic dysfunction and renal dysfunction (Am. J. Epidem. 2000, 151(9):839-845). This disorder is prevalent in over 72,000 individuals in the United States and over 2 million individuals world-wide. Over 2 million Americans are believed to carry the sickle cell allele.
Pain associated with a vaso-occlusive crisis among patients with SCD is a common reason for emergency department visits and hospitalization. Acute pain in patients with SCD is ischemic in nature and results from the occlusion of microvascular beds. Clinical data indicate that more than five percent of patients with SCD have from three to ten episodes of painful vaso-occlusive crises per year. In many patients a vaso-occlusive episode will typically be resolved in about a week. In some cases, severe episodes may persist for several weeks or even months. An ischemia-reperfusion injury can also contribute to cumulative organ damage in SCD. In addition, irreversible organ damage can result from recurrent ischemic insults and may lead to acute chest syndrome, renal hypertrophy and isosthenuria (inability to concentrate urine), autoinfarction of the spleen, chronic skin ulcers, osteonecrosis, priapism and cerebrovascular accident.
Sickle Cell Disease has historically been viewed as a disease of red cell abnormalities. Recently, it has been suggested that the wide spectrum of clinical manifestations of this disease result in part from chronic inflammation. This concept is supported by evidence that SCD patients demonstrate many clinical symptoms of chronic inflammation such as increased cytokine levels, the presence of circulating endothelial cells, increased white blood cell counts and an increase in cellular markers of leukocyte and endothelial activation.
Currently, acute sickle crises are managed primarily with analgesics. Standard treatment is palliative and consists primarily of opioids, hydration, rest and behavioral therapies. The pain associated with vaso-occlusive crisis is often under-treated due to the concerns of the physician with respect to narcotic addiction, tolerance, respiratory depression and excessive sedation.
Hydroxyurea is currently the only FDA approved drug for treating Sickle Cell Disease. Hydroxyurea is an S-phase cytotoxic drug and is used for long-term therapy. It is believed to increase the levels of hemoglobin F which prevents formation of S-polymers and red cell sickling. It is also believed to increase NO production. A multi-center trial of hydroxyurea in adults with Sickle Cell Disease showed that hydroxyurea reduced the incidence of painful episodes by nearly half. However, presently hydroxyurea is used only in patients who suffer severe complications of SCD and who are capable of following the daily dosage regimes. The general belief is that Hydroxyurea therapy is effective only if given in a structured environment with a high potential for compliance. In addition, many SCD patients are refractory to Hydroxyurea.
There is a need for new therapies for treating disorders caused by Sickle Cell Disease. Current therapies are marginally effective and have undesirable side effects.